The present invention relates to a noise immune voltage comparator with controlled hysteresis characteristics and with well defined transitions.
In a voltage comparator a voltage is compared to a reference voltage and upon coincidence therebetween the output signal of the comparator will be subjected to a transition, that is, its level will abruptly shift from one level to another. The voltage subjected to the comparison may be periodical or aperiodical and may have steep or slowly rising positive or negative sloping flank portions. The reference voltage is usually a constant DC voltage, the magnitude of which is adjustable.
A voltage comparator fundamentally comprises a fast responding amplifier, the output circuit of which is a transistor switch having output levels which are limited upwardly and downwardly and which by way of example may be the supply voltage of the circuit. In order that the output signal of the comparator shall abruptly shift from one level to the other also in response to slowly varying input signals, a very high gain of the amplifier is desired. In the following specification and claims, the expression "input signal" relates to the difference between the voltage subjected to the comparison and the reference voltage. Since an amplifier cannot have an infinite gain, there will always exist a narrow linear range of amplification within which the course of the output signal must be regarded as a linear one. If the input signal is a slowly varying one, then it will stay in and within the vicinity of said linear range for a comparatively long time.
The fact that the input signal lies within the linear range of the amplifier means an attendant risk that the amplifier will start to self-oscillate.
The fact that the input signal is within the vicinity of the linear range of the amplifier after a completed transition in the output signal from the amplifier means that the voltage comparator has a poor noise immunity. Shortly after a completed transition the noise immunity may be considered to be non-existent since at that time the input signal is very close to the high gain linear range, which implies that a very small noise signal, for example in the order of about 1 millivolt, will be highly amplified and bring about an undesired transition in the output signal from the comparator. For this reason, the amplifier is usually provided with positive feedback. Thereby the amplifier is imparted a multivibrator like course of transition which prevents the input signal from staying in the "linear range" of the amplifier for a long time, even if the input signal is slowly varying. By providing the amplifier with positive feedback, its noise immunity will increase in proportion to the introduced degree of feedback.
Since the present invention starts from voltage comparators with positive feedback, the operation of such an amplifier will at first be considered.
The output signal from the comparator will depend on what has previously happened within the comparator. For this reason said transitions in the output signal from the comparator will occur after a certain time delay in comparison to the instant in which the voltage subjected to the comparison coincides with the reference voltage. If, for example, the voltage subjected to comparison is rising and coincides with the reference voltage, then the step-like transition in the output signal will not occur until the voltage subjected to comparison has risen further above the reference voltage a specific, characteristic amount, up to a level termed the positive transition level of the comparator. If, for example, the voltage subjected to comparison is descending and coincides with the reference voltage, the step-like transition in the output signal of the comparator will not occur until the voltage subjected to comparison has descended further down a specific amount under the reference voltage, down to a level termed the negative transition level of the comparator. The voltage difference between the positive and the negative transition levels is termed the hysteresis gap of the comparator. In the voltage comparators belonging to the prior art, the positive transition level is at a higher level than the negative.
The width of the hysteresis gap is of great practical importance. It often happens that the voltage subjected to comparison is impaired by high frequency noise signals or by noise voltages, so called voltage spikes, which may be generated when, for example, a switch is turned on or off. If the amplitude of the voltage spikes is higher than the width of the hysteresis gap of the comparator so called multiple transitions may occur (compare FIGS. 3a-b). When this happens, the output signal of the comparator is subjected to multiple transitions indicating voltage coincidence between the voltage subjected to comparison and the reference voltage, while actually there is voltage coincidence between the reference voltage on the one hand and the voltage subjected to comparison plus the noise signal voltage on the other. Quite often several voltage spikes occur at a time and during unfavorable conditions several multiple transitions may occur. In order to render the comparator insensitive to noise signals, that is, in order to make it noise immune, it is suitable to use a wide hysteresis gap. On the other hand, a wide hysteresis gap means that the instant at which the transition appears in the output signal from the comparator will be much delayed in relation to the instant of voltage coincidence between the voltage subjected to the comparison and the reference voltage in case the voltage subjected to the comparison is a slowly varying one. This particular kind of delay (hysteresis) where the delay is varying with the slope of the voltage subjected to comparison may be very troublesome and non-desired, especially in such applications wherein the comparator is used as an analog-to-digital converter and minimum time delayed transitions are desired.
In constructing a voltage comparator today a compromise must be made between the following mutually contradicting qualities, namely a wide hysteresis gap which promotes the noise immunity and consolidated transitions (that is, once a transition has occurred in the output signal, the comparator will be comparatively insensitive to noise signals) also for slowly sloping flanks of the voltage subjected to comparison, and a narrow hysteresis gap which implies bad noise immunity but short time delay. The comparators of the prior art have, if using a hysteresis gap at all, a hysteresis gap which is a compromise between the above-mentioned mutually contradicting qualities.